Specific adsorption of perchlorate anions on Pt{hkl} single crystal electrodes

Abstract

The voltammetry of Pt{111}, Pt{100}, Pt{110} and Pt{311} single crystal electrodes as a function of perchloric acid concentration (0.05-2.00 M) has been studied in order to test the assertion made in recent reports by Watanabe et al. that perchlorate anions specifically adsorb on polycrystalline platinum. Such an assertion would have significant ramifications for our understanding of electrocatalytic processes at platinum surfaces since perchlorate anions at low pH have classically been assumed not to specifically adsorb. For Pt{111}, it is found that OHad and electrochemical oxide states are both perturbed significantly as perchloric acid concentration is increased. We suggest that this is due to specific adsorption of perchlorate anions competing with OHad for adsorption sites. The hydrogen underpotential deposition (H UPD) region of Pt{111} however remains unchanged although evidence for perchlorate anion decomposition to chloride on Pt{111} is reported. In contrast, for Pt{100} no variation in the onset of electrochemical oxide formation is found nor any shift in the potential of the OHad state which normally results from the action of specifically adsorbing anions. This suggests that perchlorate anions are non-specifically adsorbed on this plane although strong changes in all H UPD states are observed as perchloric acid concentration is increased. This manifests itself as a redistribution of charge from the H UPD state situated at more positive potential to the one at more negative potential. For Pt{110} and Pt{311}, marginal changes in the onset of electrochemical oxide formation are recorded, associated with specific adsorption of perchlorate. Specific adsorption of perchlorate anions on Pt{111} is deleterious to electrocatalytic activity in relation to the oxygen reduction reaction (ORR) as measured using a rotating disc electrode (RDE) in a hanging meniscus configuration. This study supports previous work suggesting that a large component of the ORR activity on platinum is governed by simple site blocking by specifically adsorbed anions and/or electrosorbed oxide.

Disentangling Catalytic Activity at Terrace and Step Sites on Selectively Ru-Modified Well-Ordered Pt Surfaces Probed by CO Electro-oxidation

“…24 However, we found the same apparent immobility during the oxidation of CO on Pt stepped surfaces in perchloric acid, whose anions are recognized by not being specifically adsorbed 75 or weakly adsorbed on {111} Pt facets. 76 This finding points out that the apparent immobility of CO during its oxidation cannot be explained on the basis of a strong competing adsorption of anions 24 or assuming OHads as a barrier for COads diffusion. 30 Regarding a possible electronic effect from Ru to the Pt sites, Baltruschat et al 36 proposed that the enthalpy of adsorption in the neighborhood of Ru is increased, and this might have implications to supply COads via diffusion from any sites to the neighboring of Ru sites, which has been proposed to be unfavorable by DFT calculation by Koper et al.…”

A Comparative Study on Gold and Platinum Dissolution in Acidic and Alkaline Media

“…It is generally assumed that perchlorates are non-adsorbing ions, although some recent works report that there is a weak adsorption of perchlorate anions on platinum. 68 There are no competitive anions in the alkaline electrolyte. As a result, the oxidation of gold starts at a much lower potential of ca.…”

Kinetic Isotope Effect as a Tool To Investigate the Oxygen Reduction Reaction on Pt‐based Electrocatalysts – Part I: High‐loading Pt/C and Pt Extended Surface

“…A KIE of approximately unity was reported for the ORR on platinum in a sulfuric acid electrolyte by this method, however, no measurements as of now have been conducted in perchloric, which poisons Pt to a much lower extent. Indeed, several reports are underlining the perchloric specific adsorption on platinum surfaces the latter impacts the Pt ORR activity to a much lower extent than SO 4 2− …”